In transcranial magnetic stimulation, an insulated wire coil is placed on the subject's scalp or skin. Brief electrical currents are passed through the coil, creating magnetic pulses that stimulate the brain. During the stimulation, participants will be asked to tense certain muscles slightly or perform other simple actions. The electrical activity of the muscle will be recorded on a computer through electrodes applied to the skin over the muscle. In most cases, the study will last less than 3 hours.